In this gallery, we highlight a few of the vertebrate skulls in the museum's comparative biological collections. The vertebrate skull has changed significantly over time, but alligators like the one pictured here are “living fossils” that have remained relatively unchanged from their first forms. The other skulls in this gallery are from mammals. Mammals derive from a branch that split from the ancestors of reptiles and birds in the Permian, over 250 million years ago. All groups of mammals have two tiny inner-ear bones, the malleus and incus (hammer and anvil). These two bones evolved from bones found in the jaws of reptiles and birds, but are now used for hearing in mammals. The platypus is a monotreme, a mammal that still lays eggs, just as our Permian ancestors did. The remainder of the mammals in the gallery are placentals, growing their babies inside the mother.